Strongly Exchange Coupled CorelShell Nanoparticles with High Magnetic Anisotropy: A Strategy toward Rare-Earth-Free Permanent Magnets

Antiferromagnetic(AFM)vertical bar ferrimagnetic(FiM) corel shell (CS) nanoparticles (NPs) of formula Co0.3Fe0.7O vertical bar Co0.6Fe2.4O4 with mean diameter from 6 to 18 nm have been synthesized through a one-pot thermal decomposition process. The CS structure has been generated by topotaxial oxidation of the core region, leading to the formation of a highly monodisperse single inverted AFM vertical bar FiM CS system with variable AFM-core diameter and constant FiM-shell thickness (similar to 2 nm). The sharp interface, the high structural matching between both phases, and the good crystallinity of the AFM material have been structurally demonstrated and are corroborated by the robust exchange-coupling between AFM and FiM phases, which gives rise to one among the largest exchange bias (H-E) values ever reported for CS NPs (8.6 kOe) and to a strongly enhanced coercive field (H-C). In addition, the investigation of the magnetic properties as a function of the AFM-core size (d(AFM)), revealed a nonmonotonous trend of both H-C and H-E, which display a maximum value for d(AFM) = 5 nm (19.3 and 8.6 kOe, respectively). These properties induce a huge improvement of the capability of storing energy of the material, a result which suggests that the combination of highly anisotropic AFM vertical bar FiM materials can be an efficient strategy toward the realization of novel rare-earth-free permanent magnets.